Austenitic heat-resistant alloy and welded structure

ABSTRACT

An austenitic heat-resistant alloy has a chemical composition of, in mass %: 0.04 to 0.14% C; 0.05 to 1% Si; 0.5 to 2.5% Mn; up to 0.03% P; less than 0.001% S; 23 to 32% Ni; 20 to 25% Cr 1 to 5% W; 0.1 to 0.6% Nb; 0.1 to 0.6% V; 0.1 to 0.3% N; 0.0005 to 0.01% B; 0.001 to 0.02% Sn; up to 0.03% AI; up to 0.02% 0; 0 to 0.5% Ti; 0 to 2% Co; 0 to 4% Cu; 0 to 4% Mo; 0 to 0.02% Ca; 0 to 0.02% Mg; 0 to 0.2% REM; and the balance being Fe and impurities. The alloy microstructure has a grain size number in accordance with ASTM E112 of 2.0 or more and less than 7.0.

TECHNICAL FIELD

The present invention relates to an austenitic heat-resistant alloy anda welded structure including this alloy.

BACKGROUND ART

In recent years, worldwide efforts have been made to increase steamtemperatures and pressures during the operation of thermal power boilersor the like to reduce loads to the environment. Materials used insuperheater tubes or reheater tubes are required to have improvedhigh-temperature strength and corrosion resistance.

To meet these requirements, various austenitic heat-resistant alloyscontaining large amounts of nitrogen have been disclosed.

For example, JP 2004-250783 A proposes an austenitic stainless steelwith improved high-temperature strength and corrosion resistance, wherethe N content is 0.1 to 0.35% and the Cr content is higher than 22% andlower than 30%, and a metallic microstructure is specified.

JP 2009-084606 A proposes an austenitic stainless steel with improvedhigh-temperature strength and corrosion resistance, where the N contentis 0.1 to 0.35% and the Cr content is higher than 22% and lower than30%, and impurity elements are specified.

JP 2012-1749 A discloses an austenitic heat-resistant steel withimproved high-temperature strength and hot workability containing 0.09to 0.30% N and having large amounts of Mo and W in composite addition.

WO 2009/044796 A1 discloses a high-strength austenitic stainless steelcontaining 0.03 to 0.35% N and one or more of Nb, V and Ti.

DISCLOSURE OF THE INVENTION

These austenitic heat-resistant alloys are usually welded for assemblyand then used at high temperatures. However, when welded structuresusing austenitic heat-resistant alloys having high N contents are usedat high temperatures for a prolonged period of time, cracks calledstrain-induced precipitation hardening (SIPH) cracks may occur inweld-heat-affected zones.

WO 2009/044796 A1 discussed above states that limiting the amounts ofthe elements that cause embrittlement of the grain boundaries and theelements that strengthen the grain interiors to certain ranges preventscracking that would occur during use for a prolonged period of time.Indeed, these materials prevent cracking under certain conditions.However, in recent years, the use of austenitic heat-resistant alloyswith large amounts of W, Mo etc. added thereto to further improveproperties such as high-temperature strength has become widespread. Forsome weld conditions, structure shapes and sizes, for example, theseaustenitic heat-resistant alloys may not prevent cracking in a stablemanner. More specifically, they may not prevent cracking in a stablemanner for high welding heat inputs, heavy plate thicknesses or high usetemperatures such as above 650° C.

An object of the present invention is to provide an austeniticheat-resistant alloy that provides good crack resistance andhigh-temperature strength in a stable manner.

An austenitic heat-resistant alloy according to an embodiment of thepresent invention has a chemical composition of, in mass %: 0.04 to0.14% C; 0.05 to 1% Si; 0.5 to 2.5% Mn; up to 0.03% P; less than 0.001%S; 23 to 32% Ni; 20 to 25% Cr; 1 to 5% W; 0.1 to 0.6% Nb; 0.1 to 0.6% V;0.1 to 0.3% N; 0.0005 to 0.01% B; 0.001 to 0.02% Sn; up to 0.03% Al; upto 0.02% O; 0 to 0.5% Ti; 0 to 2% Co; 0 to 4% Cu; 0 to 4% Mo; 0 to 0.02%Ca; 0 to 0.02% Mg; 0 to 0.2% REM; and the balance being Fe andimpurities, the alloy having a microstructure with a grain sizerepresented by a grain size number in accordance with ASTM E112 of 2.0or more and less than 7.0.

The present invention provides an austenitic heat-resistant alloy thatprovides good crack resistance and high-temperature strength in a stablemanner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a bevel produced for the Examples,showing the shape of the groove thereof.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

The present inventors conducted a detailed investigation to solve theabove-discussed problems, and revealed the following findings.

The inventors thoroughly investigated SIPH cracks occurring, during use,in welded joints using austenitic heat-resistant alloys with high Ncontents. They found that (1) cracks developed along grain boundaries inweld-heat-affected zones with coarse grains near the fusion lines, and(2) clear concentrating of S was detected on the fractured surfaces ofcracks. They further found that (3) large amounts of nitrides andcarbonitrides had precipitated within grains near the cracks. This wasparticularly significant for high Nb contents. In addition, they foundthat (4) the larger the initial grain size of the used austeniticheat-resistant alloy, the larger the grain size in weld-heat-affectedzones became and the more likely cracking occurred.

From these finding, they assumed that SIPH cracks occurred because largeamounts of nitrides and carbonitrides precipitate within grains duringuse at high temperatures and thus the grain interiors become less likelyto be deformed, which leads to concentration of creep deformations ongrain boundaries and finally to openings. S segregates on grainboundaries during welding or during use and thereby decreases thebonding force of the grain boundaries. Further, the larger the grainsize, the smaller the area of grain boundaries per unit volume. Grainboundaries work as sites for producing nuclei for nitride andcarbonitride particles. Thus, the smaller the grain boundaries, thelarger the amounts of nitrides and carbonitrides that precipitate withingrains. In addition, creep deformations that are caused by externalforces applied during use, for example welding residual stress, are themore likely to be concentrated on certain grain boundaries. Thus, theinventors concluded that the larger the initial grain size of the basematerial, the more likely cracking occurs. Particularly, they concludedthat, at high temperatures above 650° C., precipitates precipitate inshort periods of time and, in addition, grain-boundary segregationoccurs at early stages, making the problems more apparent.

To prevent such cracking, it is effective to reduce elements thatincrease the deformation resistance within the grains by usingprecipitation strengthening or solute strengthening. However, theseelements are indispensable to provide sufficient creep strength at hightemperatures. Thus, the prevention of cracks and the provision ofsufficient creep strength at high temperatures are tradeoffs and aredifficult to achieve at the same time.

After extended research, the inventors revealed that, in order toprevent SIPH cracking in an austenitic heat-resistant alloy containing0.04 to 0.14% C, 0.05 to 1% Si, 0.5 to 2.5% Mn, up to 0.03% P, 23 to 32%Ni, 20 to 25% Cr, 1 to 5% W, 0.1 to 0.3% N, 0.0005 to 0.01% B, up to0.03% Al, and up to 0.02% O, it is effective to exactly control the Nband S contents to be in the range of 0.1 to 0.6% and below 0.001%,respectively, and to have an initial grain size of the base materialrepresented by a grain size number as defined by the American Societyfor Testing and Material (ASTM) of 2.0 or more. However, if the grainsize is finer than necessary and the Nb content is limited, the creepstrength of the base material does not reach a specified value. Thus,the inventors found that the grain size as represented by grain sizenumber needs to be less than 7.0. In addition, they revealed that V,which has a lower precipitation strengthening property than Nb, in acontent of 0.1 to 0.6% is necessary to achieve a predetermined creepstrength without impairing SIPH crack resistance.

While the inventors determined that these steps indeed prevent SIPHcracking, they found out during the research that another problem mayarise.

As discussed above, austenitic heat-resistant alloys are generallywelded for assembly. When they are welded, a filler material is usuallyused. However, for small parts with thin wall thicknesses, or even forcomponents with heavy wall thickness for root running or tack welding,gas shield-arc welding may be performed without using a filler material.If the penetration depth is insufficient at this time, unwelded abuttingsurfaces remain as weld defects, and the strength required of a weldedjoint cannot be obtained. While S reduces SIPH crack resistance, S hasthe effect of increasing the penetration depth. Thus, the inventorsfound that the problem of insufficient penetration depth tends to beapparent if the S content is exactly controlled to be below 0.001% toaddress the issue of SIPH crack resistance.

To prevent insufficient penetration depth, welding heat input may besimply increased. However, increasing welding heat input brings aboutgrains coarsening in weld-heat-affected zones, and the inventors failedto prevent SIPH cracking even when the initial grain size of the basematerial had a grain size number of 2.0 or more.

After further research, the inventors found that, in order to preventinsufficient penetration depth in a stable manner, it is effective tohave an Sn content in the range of 0.001 to 0.02%. They concluded thatthis is because Sn can easily evaporate from the surface of the moltenpool during welding and ionize in the arc to contribute to the formationof an electrifying path, thereby increasing the current density of thearc.

The present invention was made based on the above-discussed findings. Anaustenitic heat-resistant alloy according to an embodiment of thepresent invention will now be described in detail.

[Chemical Composition]

The austenitic heat-resistant alloy according to the present embodimenthas the chemical composition described below. In the followingdescription, “%” in the content of an element means mass percent.

C: 0.04 to 0.14%

Carbon (C) stabilizes the austenite microstructure and forms finecarbide particles to improve creep strength during use at hightemperatures. 0.04% or more C needs to be contained in order that theseeffects are sufficiently present. However, if an excess amount of C iscontained, large amounts of carbides precipitate, which reduces SIPHcrack resistance. In view of this, the upper limit should be 0.14%. Thelower limit of C content is preferably 0.05%, and more preferably 0.06%.The upper limit of C content is preferably 0.13%, and more preferably0.12%.

Si: 0.05 to 1%

Silicon (Si) has a deoxidizing effect, and is effective in improving thecorrosion resistance and oxidation resistance at high temperatures.0.05% or more Si needs to be contained in order that these effects aresufficiently present. However, if an excess amount of Si is contained,the stability of the microstructure decreases, which decreases toughnessand creep strength. In view of this, the upper limit should be 1%. Thelower limit of Si content is preferably 0.08%, and more preferably 0.1%.The upper limit of Si content is preferably 0.6%, and more preferably0.5%.

Mn: 0.5 to 2.5%

Similar to Si, manganese (Mn) has a deoxidizing effect. Mn alsocontributes to the stabilization of austenite microstructure. 0.5% ormore Mn needs to be contained in order that these effects aresufficiently present. However, if an excess amount of Mn is contained,this causes embrittlement of the alloy, and creep ductility decreases.In view of this, the upper limit should be 2.5%. The lower limit of Mncontent is preferably 0.6%, and more preferably 0.7%. The upper limit ofMn content is preferably 2%, and more preferably 1.5%.

P: Up to 0.03%

Phosphorus (P) is contained in the alloy in the form of an impurity,and, during welding, segregates on grain boundaries inweld-heat-affected zones, thereby increasing liquation crackingsusceptibility. P also decreases creep ductility after use for aprolonged period of time. In view of this, an upper limit should be setfor P content, which should be 0.03% or lower. The upper limit of Pcontent is preferably 0.028%, and more preferably 0.025%. It ispreferable to minimize P content; however, reducing it excessivelycauses increased steel-manufacturing cost. In view of this, the lowerlimit of P content is preferably 0.0005%, and more preferably 0.0008%.

S: Less than 0.001%

Similar to P, sulfur (S) is contained in the alloy in the form of animpurity, and, during welding, segregates on grain boundaries inweld-heat-affected zones, thereby increasing liquation crackingsusceptibility. S also segregates on grain boundaries during use for aprolonged period of time and causes embrittlement, which significantlyreduces SIPH crack resistance. To prevent these effects within thelimits of the chemical composition of the present embodiment, the Scontent needs to be less than 0.001%. The upper limit of S content ispreferably 0.0008%, and more preferably 0.0005%. It is preferable tominimize S content; however, reducing it excessively causes increasedsteel-manufacturing cost. In view of this, the lower limit of S contentis preferably 0.0001%, and more preferably 0.0002%.

Ni: 23 to 32%

Nickel (Ni) is an element indispensable for providing sufficientstability of the austenite phase during use for a prolonged period oftime. 23% or more Ni needs to be contained in order that this effect issufficiently present within the limits of Cr and W contents of thepresent embodiment. However, Ni is an expensive element, and largeamounts of Ni contained mean increased costs. In view of this, the upperlimit should be 32%. The lower limit of Ni content is preferably 25%,and more preferably 25.5%. The upper limit of Ni content is preferably31.5%, and more preferably 31%.

Cr: 20 to 25%

Chromium (Cr) is an element indispensable for providing sufficientoxidation resistance and corrosion resistance at high temperatures. Cralso forms fine carbide particles to contribute to the provision ofsufficient creep strength, too. 20% or more Cr needs to be contained inorder that these effects are sufficiently present within the limits ofNi content of the present embodiment. However, if an excessive amount ofCr is contained, the microstructure stability of the austenite phase athigh temperatures deteriorates, which decreases creep strength. In viewof this, the upper limit should be 25%. The lower limit of Cr content ispreferably 20.5%, and more preferably 21%. The upper limit of Cr contentis preferably 24.5%, and more preferably 24%.

W: 1 to 5%

Tungsten (W) dissolves in the matrix, or forms fine intermetalliccompounds to significantly contribute to the improvement of creepstrength and tensile strength at high temperatures. 1% or more W needsto be contained in order that these effects are sufficiently present.However, if an excess amount of W is contained, the deformationresistance with grains becomes high and SIPH crack resistance reduces,and creep strength may decrease. Further, W is an expensive element, andlarge amounts of W contained mean increased costs. In view of this, theupper limit should be 5%. The lower limit of W content is preferably1.2%, and more preferably 1.5%. The upper limit of W content ispreferably 4.5%, and more preferably 4%.

Nb: 0.1 to 0.6%

Niobium (Nb) precipitates in the form of fine MX carbonitride particles,and, in addition, precipitates in the form of Z phase (CrNbN) withingrains to significantly contribute to the improvement of creep strengthand tensile strength at high temperatures. 0.1% or more Nb needs to becontained in order that these effects are sufficiently present. However,if an excess amount of Nb is contained, the strengthening property ofthese precipitates is too high, which reduces SIPH crack resistance andcauses a decrease in creep ductility and toughness. In view of this, theupper limit should be 0.6%. The lower limit of Nb content is preferably0.12%, and more preferably 0.15%. The upper limit of Nb content ispreferably 0.55%, and more preferably 0.5%.

V: 0.1 to 0.6%

Vanadium (V) precipitates in the form of fine MX carbonitride particleswithin the grains to contribute to the improvement of creep strength andtensile strength at high temperatures. 0.1% or more V needs to becontained in order that these effects are sufficiently present. However,if an excess amount of V is contained, large amounts of carbonitridesprecipitate, which reduces SIPH crack resistance and causes a decreasein creep ductility and toughness. In view of this, the upper limitshould be 0.6%. The lower limit of V content is preferably 0.12%, andmore preferably 0.15%. The upper limit of V content is preferably 0.55%,and more preferably 0.5%.

N: 0.1 to 0.3%

Nitrogen (N) stabilizes the austenite microstructure, and dissolves inthe matrix or precipitates in the form of nitrides to contribute to theimprovement of high-temperature strength. 0.1% or more N needs to becontained in order that these effects are sufficiently present. However,if an excessive amount of N is contained, it dissolves during use for ashort period of time, or large amounts of fine nitride particlesprecipitate within grains during use for a prolonged period of time,thereby increasing the deformation resistance within grains, whichreduces SIPH crack resistance. Further, creep ductility and toughnessdecrease. In view of this, the upper limit should be 0.3%. The lowerlimit of N content is preferably 0.12%, and more preferably 0.14%. Theupper limit of N content is preferably 0.28%, and more preferably 0.26%.

B: 0.0005 to 0.01%

Boron (B) provides fine dispersed grain-boundary carbide particles toimprove creep strength, and segregates on grain boundaries to strengthengrain boundaries. 0.0005% or more B needs to be contained in order thatthese effects are sufficiently present. However, if an excess amount ofB is contained, the weld thermal cycle during welding causes a largeamount of B to segregate in weld heat affected zones near meltboundaries to decrease the melting point of grain boundaries, therebyincreasing liquation cracking susceptibility. In view of this, the upperlimit should be 0.01%. The lower limit of B content is preferably0.0008%, and more preferably 0.001%. The upper limit of B content ispreferably 0.008%, and more preferably 0.006%.

Sn: 0.001 to 0.02%

Tin (Sn) has the effect of increasing the penetration depth duringwelding by evaporating from the molten pool to increase the currentdensity of the arc. 0.001% or more Sn needs to be contained in orderthat these effects are sufficiently present. However, if an excessamount of Sn is contained, the liquation cracking susceptibility inweld-heat-affected zones during welding and the SIPH cracksusceptibility during use become high. In view of this, the upper limitshould be 0.02%. The lower limit of Sn content is preferably 0.0016%,and more preferably 0.002%. The upper limit of Sn content is preferably0.018%, and more preferably 0.015%.

Al: Up to 0.03%

Aluminum (Al) has a deoxidizing effect. However, if an excess amount ofAl is contained, the cleanliness of the alloy deteriorates, whichdecreases hot workability. In view of this, the upper limit should be0.03%. The upper limit of Al content is preferably 0.025%, and morepreferably 0.02%. No lower limit needs to be set; still, it should benoted that decreasing Al excessively causes an increase insteel-manufacturing cost. In view of this, the lower limit of Al contentis preferably 0.0005%, and more preferably 0.001%. Al as used hereinmeans acid-soluble Al (sol. Al).

O: Up to 0.02%

Oxygen (O) is contained in the alloy in the form of an impurity, and hasthe effect of increasing the penetration depth during welding. However,if an excess amount of O is contained, hot workability decreases andtoughness and ductility deteriorate. In view of this, the upper limitshould be 0.02%. The upper limit of O content is preferably 0.018%, andmore preferably 0.015%. No lower limit needs to be set; still, it shouldbe noted that decreasing O excessively causes an increase insteel-manufacturing cost. In view of this, the lower limit of O contentis preferably 0.0005%, and more preferably 0.0008%.

The balance of the chemical composition of the austenitic heat-resistantalloy in the present embodiment is Fe and impurities. Impurity as usedherein means an element originating from ore or scrap used as rawmaterial for the heat-resistant alloy being manufactured on anindustrial basis or an element that has entered from the environment orthe like during the manufacturing process.

In the chemical composition of the austenitic heat-resistant alloy inthe present embodiment, some of the Fe may be replaced by one or moreelements selected from one of the first to third groups provided below.All of the elements listed below are optional elements. That is, none ofthe elements listed below may be contained in the austeniticheat-resistant alloy of the present embodiment. Or, only one or some ofthem may be contained.

More specifically, for example, only one group may be selected fromamong the first to third groups and one or more elements may be selectedfrom this group. In this case, it is not necessary to select all theelements belonging to the selected group. Further, a plurality of groupsmay be selected from among the first to third groups and one or moreelements may be selected from each of these groups. Again, it is notnecessary to select all the elements belonging to the selected groups.

First Group—Ti: 0 to 0.5%

The element belonging to the first group is Ti. Ti improves the creepstrength of the alloy through precipitation strengthening.

Ti: 0 to 0.5%

Similar to Nb and V, Titanium (Ti) combines with carbon or nitrogen toform fine carbide or carbonitride particles, thereby contributing to theimprovement of creep strength. These effects are present if a smallamount of Ti is contained. On the other hand, if an excess amount of Tiis contained, large amounts of precipitates are produced, which reducesSIPH resistance and creep ductility. In view of this, the upper limitshould be 0.5%. The lower limit of Ti content is preferably 0.01%, andmore preferably 0.03%. The upper limit of Ti content is preferably0.45%, and more preferably 0.4%.

Second Group—Co: 0 to 2%, Cu: 0 to 4%, Mo: 0 to 4%

The elements belonging to the second group are Co, Cu, and Mo. Theseelements improve the creep strength of the alloy.

Co: 0 to 2%

Similar to Ni, cobalt (Co) is an austenite-forming element, andincreases the stability of the austenite microstructure to contribute tothe improvement of creep strength. These effects are present if a smallamount of Co is contained. However, Co is a very expensive element, andlarge amounts of Co contained mean increased costs. In view of this, theupper limit should be 2%. The lower limit of Co content is preferably0.01%, and more preferably 0.03%. The upper limit of Co content ispreferably 1.8%, and more preferably 1.5%.

Cu: 0 to 4%

Similar to Ni and Co, copper (Cu) stabilizes the austenitemicrostructure, and precipitates in the form of fine particles duringuse to contribute to the improvement of creep strength. These effectsare present if a small amount of Cu is contained. On the other hand, ifan excessive amount of Cu is contained, this causes a decrease in hotworkability. In view of this, the upper limit should be 4%. The lowerlimit of Cu content is preferably 0.01%, and more preferably 0.03%. Theupper limit of Cu content is preferably 3.8%, and more preferably 3.5%.

Mo: 0 to 4%

Similar to W, molybdenum (Mo) dissolves in the matrix and contributes tothe improvement of creep strength and tensile strength at hightemperatures. These effects are present if a small amount of Mo iscontained. On the other hand, if an excessive amount of Mo is contained,the deformation resistance within grains becomes high and SIPH crackresistance reduces, and creep strength may decrease. Further, Mo is anexpensive element, and large amounts of Mo contained mean increasedcosts. In view of this, the upper limit should be 4%. The lower limit ofMo content is preferably 0.01%, and more preferably 0.03%. The upperlimit of Mo content is preferably 3.8%, and more preferably 3.5%.

Third Group—Ca: 0 to 0.02%, Mg: 0 to 0.02%, REM: 0 to 0.2%

The elements belonging to the third group are Ca, Mg and REM. Theseelements improve hot workability of the alloy.

Ca: 0 to 0.02%

Calcium (Ca) improves hot workability during manufacture. This effect ispresent if a small amount of Ca is contained. On the other hand, if anexcessive amount of Ca is contained, it combines with oxygen tosignificantly decrease the cleanliness of the alloy, which decreases hotworkability. In view of this, the upper limit should be 0.02%. The lowerlimit of Ca content is preferably 0.0005%, and more preferably 0.001%.The upper limit of Ca content is preferably 0.01%, and more preferably0.005%.

Mg: 0 to 0.02%

Similar to Ca, magnesium (Mg) improves hot workability duringmanufacture. This effect is present if a small amount of Mg iscontained. On the other hand, if an excess amount of Mg is contained, itcombines with oxygen to significantly decrease the cleanliness of thealloy, which decreases hot workability. In view of this, the upper limitis 0.02%. The lower limit of Mg content is preferably 0.0005%, and morepreferably 0.001%. The upper limit of Mg content is preferably 0.01%,and more preferably 0.005%.

REM: 0 to 0.2%

Similar to Ca and Mg, rare-earth metals (REMs) improve hot workabilityduring manufacture. This effect is present if a small amount of REM iscontained. On the other hand, if an excessive amount of REM iscontained, it combines with oxygen to significantly decrease thecleanliness of the alloy, which decreases hot workability. In view ofthis, the upper limit should be 0.2%. The lower limit of REM content ispreferably 0.0005%, and more preferably 0.001%. The upper limit of REMcontent is preferably 0.15%, and more preferably 0.1%.

“REM” is a collective term for a total of 17 elements, i.e. Sc, Y andthe lanthanoids, and “REM content” means the total content of one ormore REM elements. REMs are usually contained in mischmetal. Thus, forexample, mischmetal may be added to the alloy such that the REM contentis in the above-indicated range.

Particularly, Nd has a strong affinity for S and P, and has the effectof reducing weld liquation cracking susceptibility by forming sulfidesor phosphides, and thus it is more preferable to utilize Nd.

[Microstructure]

Grain Size Number: 2.0 or More and Less than 7.0

The austenitic heat-resistant alloy according to the present embodimenthas a microstructure having a grain size represented by a grain sizenumber in accordance with ASTM E112 of 2.0 or more and less than 7.0.

In order to give sufficient SIPH crack resistance to theweld-heat-affected zones of a welded structure using the austeniticheat-resistant alloy of the present embodiment, the grains of themicrostructure before welding need to be fine grains, i.e. their size asrepresented by grain size number in accordance with ASTM E112 needs tobe 2.0 or more, in order to prevent the grains in the weld-heat-affectedzones from becoming excessively coarse even after being affected by theheat cycle from the welding. On the other hand, if the grains are sofine as to have a grain size number of 7.0 or more, the required creepstrength is not obtained. In view of this, the grain size number shouldbe 2.0 or more and less than 7.0.

The microstructure having the above-specified grain size can be providedby performing a heat treatment on the alloy with the above-specifiedchemical composition under appropriate conditions. This microstructuremay be achieved by, for example, shaping the alloy of theabove-specified chemical composition into a predetermined shape by hotworking or cold working before performing a solution heat treatment inwhich it is held at temperatures of 900 to 1250° C. for 3 to 60 minutesbefore water cooling. The higher the holding temperature of the solutionheat treatment and the longer the holding time, the larger the grainsize becomes (i.e. the smaller the grain size number becomes). Morepreferably, the solution heat treatment involves holding the alloy attemperatures of 1120 to 1220° C. for 3 to 45 minutes before watercooling, and yet more preferably holding the alloy at temperatures of1140 to 1210° C. for 3 to 30 minutes before water cooling.

The austenitic heat-resistant alloy according to an embodiment of thepresent invention has been described. The present embodiment provides anaustenitic heat-resistant alloy providing good crack resistance andhigh-temperature strength in a stable manner.

EXAMPLES

The present invention will be described in more detail below usingexamples. The present invention is not limited to these examples.

The materials labeled A to J having the chemical compositions shown inTable 1 were melted in a laboratory and ingots were cast, which weresubjected to hot forging and hot rolling in the temperature range of1000 to 1150° C. to provide plates with a thickness of 20 mm. Theseplates were further subjected to cold rolling to the thickness of 16 mm.The plates were subjected to a solution heat treatment in which theywere held at 1200° C. for a predetermined period of time before watercooling. After the solution heat treatment, they were machined to plateswith a thickness of 14 mm, a width of 50 mm and a length of 100 mm. Fromother plates subjected to the solution heat treatment, samples to beused for microstructure observation were taken and the grain size of themicrostructure of each sample was measured in accordance with ASTM E112. From material A, materials with different grain sizes were producedby changing the holding time of the solution heat treatment in the rangeof 3 to 30 minutes.

TABLE 1 Chemical composition (in mass %. balance being Fe andimpurities) Mark C Si Mn P S Ni Cr W Nb V N B Al O Sn Other A 0.09 0.280.98 0.017 0.0008 30.2 21.8 3.3 0.25 0.21 0.197 0.0023 0.005 0.009 0.004B 0.08 0.32 1.02 0.008 0.0006 28.5 22.0 3.0 0.23 0.22 0.206 0.0017 0.0060.008 0.012 Nd: 0.023 C 0.10 0.25 1.10 0.016 0.0005 27.1 21.7 2.7 0.180.19 0.174 0.0018 0.005 0.009 0.001 Ti: 0.12, Ca: 0.002, Cu: 0.41, Mo:0.03 D 0.07 0.34 1.18 0.014 0.0004 30.6 22.3 2.9 0.21 0.19 0.185 0.00260.004 0.010 0.016 Nd: 0.015, Co: 0.08, Mg: 0.001 E 0.07 0.29 0.82 0.0170.0002 29.8 22.4 2.8 0.22 0.21 0.211 0.0024 0.012 0.004 — * F 0.11 0.290.96 0.021  0.0021 * 30.5 21.9 3.1 0.38 0.31 0.198 0.0015 0.007 0.008— * Ti: 0.18 G 0.09 0.30 0.98 0.023 0.0003 30.3 22.0 2.7 0.42 0.29 0.2150.0044 0.003 0.009  0.033 * Nd: 0.010 H 0.08 0.25 0.95 0.015 0.0008 22.4 * 24.6 2.5 0.45 0.21 0.221 0.0024 0.004 0.010 0.010 I 0.08 0.251.04 0.015 0.0007 30.9 22.0 3.1 0.24 0.20 0.188 0.0019 0.004 0.008 0.003REM: 0.018 J 0.07 0.26 0.85 0.015 0.0006 25.6 24.5 2.2 0.16  0.08 *0.165 0.0018 0.006 0.009 0.004 * indicates that the value is outisde therange specified by the present invention.

[Weldability]

The groove shown in FIG. 1 was provided along the longitudinal directionof each plate produced as described above. With grooved plates abuttingeach other, two joints for each mark were subjected to butt weldingusing gas-tungsten arc welding to produce welded joints. The welding didnot use filler material, and the amount of heat input was 5 kJ/cm.

Those of the obtained welded joints that had back beads with a width of2 mm or more across the entire length of the weld line for both jointparts were determined to have good weldability in fabrication and thusto have “passed” the test. Those that had a portion for either jointpart in which no back bead was present were determined to have poorweldability in fabrication and thus to be “unacceptable”.

[Weld Crack Resistance]

Each of the above-described welded joints, with only a first weldedlayer (i.e. root running), was placed on a commercial steel plateequivalent to the SM400B plate specified by JIS G 3106 (2008) (with athickness of 30 mm, a width of 200 mm and a length of 200 mm), andrestraint welding was performed on the four sides using a covered arcwelding rod ENi 6625 specified by JIS Z 3224 (2010). Thereafter, a tigwire equivalent to the SNi 6625 wire specified by JIS Z 3334 (2011) wasused to perform a multi-layer welding in the groove by TIG welding witha heat input of 10 to 15 kJ/cm, thereby producing welded joints, two foreach mark.

Aging was performed on one of the welded-joint parts for each mark at700° C. for 500 hours. Samples were taken from five points on each ofthe as-welded joints and welded joints after aging, with the observationsurface represented by a transverse cross section of the joint (i.e.cross section perpendicular to the weld bead). Mirror polishing andetching were performed on these samples before inspection by opticalmicroscopy to determine whether cracks were present in theweld-heat-affected zones. Welded joints where no cracks were found inany of the five samples were determined to be “good” and those wherecracks were found in one sample were determined to be “acceptable”, thusto have passed the test. Those welded joints where cracks were found intwo or more samples were determined to be “unacceptable”.

[Creep-Rupture Strength]

From those as-welded joints that have passed the weld crack resistancetest, round-bar creep-rupture test specimens were taken such that thecenter of the parallel portion was made of welded metal. Creep-rupturetesting was conducted at 700° C. and under 167 MPa, conditions thatresult in a target fracture time for the base material of about 1000hours. The base material was fractured, and those joints where thefracture time was 90% or more of the fracture time of the base material(i.e. 900 hours or longer) were determined to have “passed” the test.

[Performance Evaluation Results]

The performance evaluation results are shown in Table 2. Table 2 alsoshows the grain size number of the austenitic heat-resistant alloy foreach mark.

TABLE 2 Grain Creep- size Weldability Weld crack rupture Mark number inFabrication as-welded aged test result A-1 2.3 passed good good passedA-2 3.7 passed good good passed A-3 5.4 passed good good passed A-4 6.8passed good good passed A-5  1.7 * passed acceptable unacceptable nottested A-6  7.5 * passed good good not passed B 3.5 passed good goodpassed C 3.4 passed good good passed D 3.6 passed acceptable acceptablepassed E 3.1 unacceptable good good passed F 3.6 passed acceptableunacceptable not tested G 3.4 passed unacceptable unacceptable nottested H 3.8 passed good good not passed I 3.5 passed good good passed J3.2 passed good good not passed * indicates that the value is outisdethe range specified by the present invention.

Each of the welded joints using the austenitic heat-resistant alloyswith Marks A-1 to A-4, B to D and I as the base material had anappropriate chemical composition, where the initial grain size of thebase material had a grain size of 2.0 or more and less than 7.0. Each ofthese welded joints had a back bead across the entire length after rootrunning, and had good weldability in fabrication. Further, though thethickness of the base material was 14 mm, which is relatively large, nocracks were produced in weld-heat-affected zones even after aging,meaning good crack resistance. Further, the creep-rupture strength athigh temperatures was sufficient.

In the welded joint using the austenite heat-resistant alloy with MarkA-5 as the base material, cracks that are believed to be SIPH crackswere produced after aging. This is presumably because the grain size ofthe austenitic heat-resistant alloy with Mark A-5 was too large.

The welded joint using the austenitic heat-resistant alloy with Mark A-6as the base material had good crack resistance, but the creep-rupturetime was below the target. This is presumably because the grain size ofthe austenitic heat-resistant alloy with Mark A-6 was too small.

In the welded joint using the austenitic heat-resistant alloy with MarkE as the base material, no back bead was present in some portions afterroot running. This is presumably because the Sn content of theaustenitic heat-resistant alloy with Mark E was too low.

The welded joint using the austenitic heat-resistant alloy with Mark Fas the base material contained no Sn but a large amount of S such that asufficient back bead was produced. However, cracks that are believed tobe SIPH cracks were produced after aging.

In the welded joint using the austenitic heat-resistant alloy with MarkG as the base material, directly after welding and after aging, cracksthat are believed to be liquation cracks and SIPH cracks, respectively,were produced. This is presumably because the Sn content of theaustenitic heat-resistant alloy with Mark G was too high.

In the welded joint using the austenitic heat-resistant alloy with MarkH as the base material, the weldability in fabrication and weld crackresistance were good but the required creep strength was not satisfied.This is presumably because the Ni content of the austeniticheat-resistant alloy with Mark H was too low, impairing phase stability.

In the welded joint using the austenitic heat-resistant alloy with MarkJ as the base material, too, the required creep strength was notsatisfied. This is presumably because the amount of V contained in theaustenitic heat-resistant alloy with Mark J was lower than the lowerlimit.

INDUSTRIAL APPLICABILITY

The present invention can be suitably used as an austeniticheat-resistant alloy used as a high-temperature part such as a mainsteam tube or high-temperature reheating steam tube in a thermal powerboiler.

1. An austenitic heat-resistant alloy having a chemical composition of,in mass %: 0.04 to 0.14% C; 0.05 to 1% Si; 0.5 to 2.5% Mn; up to 0.03%P; less than 0.001% S; 23 to 32% Ni; 20 to 25% Cr; 1 to 5% W; 0.1 to0.6% Nb; 0.1 to 0.6% V; 0.1 to 0.3% N; 0.0005 to 0.01% B; 0.001 to 0.02%Sn; up to 0.03% Al; up to 0.02% O; 0 to 0.5% T; 0 to 2% Co; 0 to 4% Cu;0 to 4% Mo; 0 to 0.02% Ca; 0 to 0.02% Mg; 0 to 0.2% REM; and the balancebeing Fe and impurities, the alloy having a microstructure with a grainsize represented by a grain size number in accordance with ASTM E112 of2.0 or more and less than 7.0.
 2. The austenitic heat-resistant alloyaccording to claim 1, wherein the chemical composition contains one ormore elements selected from one of the first to third groups providedbelow, in mass %: first group: 0.01 to 0.5% Ti; second group: 0.01 to 2%Co, 0.01 to 4% Cu, and 0.01 to 4% Mo; and third group: 0.0005 to 0.02%Ca; 0.0005 to 0.02% Mg; and 0.0005 to 0.2% REM.
 3. A welded structureincluding the austenitic heat-resistant alloy according to claim
 1. 4. Awelded structure including the austenitic heat-resistant alloy accordingto claim 2.